Previous work has led to the description of a mechanism for the transport of weak electrolytes in the small intestine. It is suggested that this mechanism may play a significant role in the transport of a wide variety of compounds of pharmacological importance in the intestine, and at other epithelia in the body. A broadly based study is proposed in which will be investigated the mechanisms for transport of pharmacologically active, and other weak electrolytes, with the objective of establishing specific empirical, and general theoretical bases for the description of the determinants of drug transport in epithelia. The compounds to be studied will include analgesics, narcotics, narcotic antagonists, autonomic effector agents and other drugs. Particular attention will be paid to the transport of these compounds in tissues such as the gallbladder, intestine, and stomach or amphibian urinary bladder, that are considered to be representative models of other epithelia, but studies of drug transport in the choroid plexus, cornea, pancreas, and liver are also included in the proposal.